Industrial lubricant base oil is a mixture of several branched alkane, obtained by petroleum cracking or α-olefin oligomerization (PAO). Wherein, PAO, as a series of very important and promising lubricant base oil, is produced by α-olefin oligomerization, and the main raw material is expensive α-olefin such as α-octene, α-decene, α-dodecene and the like.
So in order to get high quality base oil PAO, α-olefin (particularly α-decene), must be obtained firstly by ethylene oligomerization with catalysts. It is difficult to produce α-olefin with more than C6 selectively, and it is economical and efficient to produce high quality base oil directly from cheap olefins such as ethylene, propylene, butene and the like. However, there is no significant progress in this area because of lacking efficient catalytic system.
Before 1995, nickel complex was believed to only catalyze oligomerization of olefin, for example the well-known SHOP catalyst could effectively catalyze ethylene to produce a series of α-olefin with Flory distribution. In 1995, through α-diimine nickel complexes, Brookhart et al. (J. Am. Chem. Soc. 1995, 117, 6414.) firstly proved that the property of the active center could be controlled by changing the structure of a ligand, thereby obtaining branched polyethylene with high molecular weight by ethylene polymerization catalyzed by nickel complexes. The melting point (Tm) of the polymer is 39° C. to 132° C., lower than common polyethylene resin. Respecting to this technology, Du pont has applied several patents (WO 96/23010, WO 98/03521, WO 98/40374, WO 99/05189, WO 99/62968, WO 00/06620, U.S. Pat. No. 6,103,658, U.S. Pat. No. 6,660,677) to protect such kind of polymer products. The corresponding palladium system could get highly branched oily polymer, however the activity of this system is relatively low. Besides, it is known that this catalyst results in severe β-H elimination. In the presence of the catalyst, β-H elimination producing carbon-carbon double bond and Pd—H are the main way of such catalytic circle, so the unsaturation is high (the bromine number is high).
The morphology and performance of polyethylene is closely related to the branching degree, and the structure of catalysts is the core to control the structure of polyethylene. The polyethylene produced by Brookhart et al. with nickel catalysts already show a certain degree of branching, but still could not meet the requirement for application of such as lubricant base oil, because the polymer products are solid.
Sen et al. found (J. Am. Chem. Soc. 1998, 120, 1932.) that Ni(II), Pd(II)/AlCl3 could catalyze ethylene polymerization to make highly branched oily polyethylene. However, the viscosity index of the oil is relatively low and not suitable for lubricant base oil. They also found (J. Am. Chem. Soc. 2000, 122, 1867.) that TaCl5, TiCl4/alkyl aluminium chloride could catalyze ethylene polymerization to make oily polymer, basically without methyl branch. Respecting to this technology, they applied several patents (WO 98/33823, WO 99/47627) to protect the products and the polymerizing method.
Industrial synthetic lubricant needs to meet the requirement of maintaining the viscosity in a large temperature range, which means having a high viscosity index. Also lubricant needs a low pour point, which is to be equal to or lower than the third class oil (group III base oil). BI, the branching degree of the polymer, is connected with these properties of the oil. BI is the ratio of methyl hydrogen number to all alkyl hydrogen number. The methyl hydrogen is characterized as in 0.5-1.05 part of HNMR, and all the alkyl hydrogen is characterized as in 0.5-2.1 part of HNMR. Generally, the pour point of lubricant oil decreases as BI increases, which means the temperature at which the lubricant oil changes from liquid to solid decreases, and the decreasing of the pour point is good for expanding the application of lubricant. However, the increase of BI generally results in the decrease of oil viscosity index, which is unfavorable for the application of lubricant. Therefore, synthetic lubricant aims to ensure the oil remaining in a liquid state under lower temperature, and have a high viscosity index, for example, maintaining a high viscosity at high temperature, such as 100° C.
In summary, there is no promising system to produce highly branched alkane directly from olefin, such as ethylene. Therefore, it is urgent to develop a high-efficient method for preparing highly branched oily polymer directly from low-cost olefin such as ethylene and the corresponding catalytic system.